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On the Trails: Plants that supplement their income

Posted: August 17, 2012 - 12:00am
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Sudetan lousewort.            Photo by Bob Armstrong
Photo by Bob Armstrong
Sudetan lousewort.

Green plants are called “autotrophs,” meaning that they feed themselves . (This is in contrast to all animals, which are “heterotrophs” that gain nutrition by consuming other organisms.) These green plants feed themselves by photosynthesis, converting carbon dioxide and water into sugars (and oxygen). They also draw water and minerals from soils, and sometimes from water. So we are inclined to think of them as functionally independent entities, in terms of nutrition.

There are some salient exceptions to this simple plant-autotroph versus animal-heterotroph dichotomy. The carnivorous plants consume insects as a dietary supplement, so they are, in effect, both autotrophic and heterotrophic. A few plants are not green at all and live a totally parasitic existence, drawing nutrition from host plants; they could be called heterotrophic too. For example, dwarf mistletoe that infects hemlocks and other conifers in our forests is not capable of much photosynthesis, and depends on its host tree for nutrition. Heavy infestations can kill the host tree. (However, the witches’ brooms that they create are useful to squirrels and birds). Northern ground cone, which is common near the Visitor Center at the glacier, is parasitic on the roots of alders (and a favorite food of local bears).

However, most of the other, supposedly autotrophic, plants actually live in association with other organisms that supply nutrients. Many species, including orchids and blueberries, associate with fungi that supply important minerals to the plant; these associations are called mycorrhizal (fungus-root). Some species, such as alders and lupines, form root nodules that are inhabited by nitrogen-fixing bacteria that turn atmospheric nitrogen into a form usable by plants. Many trees form natural root grafts with their neighbors, drawing water and nutrition from each other (and sometimes diseases too).

Then there are the so-called hemi-parasitic plants, which I mentioned a couple of weeks ago in this space. They are green and can photosynthesize carbohydrates and live independently, but which also commonly parasitize other plants. They often grow better and set more seeds when they tap a host’s resources, but a host is not absolutely necessary. Their effect on host plants is generally negative, reducing growth and seed production. As far as I can determine (so far), we have three kinds of hemi-parasitic flowering plants in our flora.

Indian paintbrush (genus Castilleja; about twelve species in Alaska): They grow from sea level to the alpine zone. The colorful bracts of the inflorescences range in color from red to pink to yellow. Some are pollinated by hummingbirds, some by butterflies (especially Down South) and some are pollinated by bumblebees. Paintbrushes can accumulate selenium from soils and become toxic to humans and other vertebrates. They parasitize the roots of grasses, herbs, and some trees.

Yellow rattle (genus Rhinanthus; one species here): It is also known as rattlebox or rattlepod. The yellow flowers are bee-pollinated. When the petals drops off, after pollination, the remaining green calyx contains the loose (rattling) seeds. A root parasite mostly of grasses and legumes, it is known to decrease the productivity and survival of grasses. Therefore it is used in some regions to restore meadows and prairies where cultivated grasses have been grown; by decreasing the cover of grasses, there is more room for wild flowers and thus a diverse community of plants. And the helpful yellow rattle plants eventually get shaded out.

Louseworts (genus Pedicularis; about twenty species in Alaska): Louseworts have their unfortunate name because of an old, very silly, belief that they caused grazing cows to have lice. There are hundreds of species of lousewort in the world, with flower colors of all hues. Most are pollinated by bumblebees or other relatively large bees, but at least one is also pollinated by hummingbirds. Only some have nectar in the flowers. Louseworts are root parasites, often of members of the heath family, such as blueberries.

The bottom line of all this is that green, flowering plants are not such independent entities as one might think. Many, if not most, of them interact with other plants, fungi, or bacteria to supplement their nutrition. Our forests and meadows would be impoverished without these interactions.

 

• Mary F. Willson is a retired professor of ecology.

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